The objective of this project is to study the pathogenesis of rabbit coronavirus (RbCV), the rabbit's physiologic response to this virus, and the relatedness of RbCV to other members of the Coronaviridae. RbCV antiserum was shown to cross react strongly with transmissible gastroenteritis virus (TGE), canine coronavirus (CCV), and feline infectious peritonitis (FIP) in the RIA, slightly to TGE and CCV, but not FIP in a plaque serum neutralization and not at all in a gel diffusion test. Two of 3 rabbits vaccinated with CCV and l of 3 vaccinated with FIP survived challenge with RbCV. None vaccinated with TGE or calf diarrhea coronavirus (CDCV) survived challenge. Single rabbits given RbCV reacted with FIP antisserum or TGE antiserum did not survive. Based on previous work showing a 2 way cross with coronavirus 229E and this new data, RbCV is most probably in Group II of the Coronaviridae [229E (Human), TGE (swine), FIP (cat), CCV (dog)]. Nucleotide homology studies will be required to prove this supposition. Such studies are hampered by the present inability to grow this virus in tissue culture. Two of 3 rabbits given RbCV reacted with CCV antiserum for 2 hours survived. The apparent protective effect of CCV vaccination will be explored further. Additional studies to understand the relatedness of RbCV to the other coronaviruses will be done. Assessment of myocardial damage using ECGs and creatine kinase isozymes are in progress. In cooperation with a team of virologists, further attempts will be made to adapt RbCV to tissue culture. The significance of this work lies in the ability to study a viral disease with a cardiotropism in an animal of sufficient but manageable size to allow sequential clinical and physiological observations. The damage to the rabbit heart by RbCV has a corollary in the human heart with the Coxsackie viruses, Mycoplasma pneumoniae, influenza virus, Herpes zoster, and possibly over infectious agents.